The interferometer is an essential tool for precisely measuring the displacement, length, etc., of the target device. In the interferometer, the change in the length of the optical path is converted into displacement so as to precisely measure the displacement. The double-frequency laser has the advantages of high resolution, fast velocity measurement, large measurement scope, and capable of carrying out multi-axis synchronous measurement, and is widely used in the advanced manufacturing and nanotechnology, for example, for positioning and measuring the workpiece stage and the reticle stage of a mask aligner with high precision.
In order to measure the length, displacement, axial rotation, and many other degrees of freedom of the target device at the same time, a multi-axis interferometer including a plurality of laser beams can be used, with each laser beam corresponding to a measurement axis of the interferometer. In the multi-axis interferometer, the multi-axis light splitting beams must have equal energy and be parallel to each other. The quality of the design of the light splitting system is the key for the multi-axis splitting interferometer. A good light splitting system enables the interferometer to have high stability and consistent temperature drift of the light beams of the light paths.
Although the multi-axis interferometer has already been successfully applied in many fields, currently, the constantly pursuing goal is to continuously improve its performance so as to obtain an excellent measurement accuracy, especially to constantly improve the light splitting system of the multi-axis interferometer so as to obtain a good stability, a low temperature drift, nonlinear errors and adjustability. Therefore, the light splitting system of the multi-axis interferometer must be carefully designed in order to minimize the measurement error caused by the imbalance of the light path, such as thermal drift, nonlinear errors, etc. Currently, the multi-axis interferometer generally uses a block optical light splitting component, which is coated with a plurality of coatings of different requirements on a single surface, for splitting light. This light splitting method requires high precision in optical processing, and the same light splitting block need to be coated with a plurality of coatings of different requirements (such as anti-reflection, full-reflection, 50% of light splitting film, etc.) on the two light surfaces, which poses great difficulties for coating film. In addition, since the light splitting beams of the light paths have different paths in the block optical light splitting component, the temperature drift of the light beams of the light paths is inconsistent. This structure will generally also causes the difference in the transmission distances of the measuring beam and the reference beam in the medium (such as quartz glass). Furthermore, since the geometrical position between each light splitting surface and reflecting surface in the block optical light splitting component is fixed, each light splitting beam cannot be adjusted separately. Therefore, such light splitting system has the disadvantages of poor consistence of the temperature drift of the light beams of the light paths, difficulties in the adjustment of the light path, etc., in the application. Similarly, the general differential interferometer uses the 45-degree block light splitting component to send the reference light to the reference reflecting mirror. Since the measurement beam and the reference beam have different paths in the optical component, it will cause different temperature drifts and measurement errors.